Are there any smart workarounds to disable autocorrect automatically -- or near automatic -- while using an external keyboard on iOS?
By default iOS provides no way to do this as seen here on the Apple forums.
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Not without a jailbreak. The system doesn't change settings based on the presence of an external keyboard.
I don't see a way for workflow or another automation app to script a change to that setting unless/until apple exposes that setting to Siri control. "Siri, turn off auto correct setting". On iOS 9 Siri knows that is a setting, but can't automate it.